This invention relates to microemulsions comprising water and hydrofluoroethers.
Emulsions are typically systems that consist of two or more phases of immiscible liquids, such as oil and water. Typically there are two phases contained in an emulsion (although multiple emulsions do exist): a dispersed phase, consisting of one liquid broken into droplets, and a continuous phase, consisting of the other liquid surrounding the droplets of the dispersed phase.
When two immiscible liquids such as a hydrocarbon oil and water are combined, the water will separate to the bottom of the vessel because it has the higher density. If the oil is a fluorocarbon, it will separate to the bottom of the vessel because it is more dense than water. This immiscibility is caused by high interfacial tension between the oil and the water.
In general, emulsion formation requires that an emulsifying agent (a surfactant) and energy be added to the system. This energy can be in the form of mechanical agitation, ultrasound, and/or heat. The surfactant acts to reduce the interfacial tension between the two phases; this allows an increase in the interfacial area, and increases the number of droplets of dispersed phase in a volume of the continuous phase.
For some applications it is desirable that these droplets be finely and uniformly distributed throughout the continuous phase. This can be accomplished through formation of microemulsions, which generally have particle sizes less than 100 nm. This small particle size provides a homogeneous, transparent or nearly transparent mixture that appears to be a solution. However, a dispersed phase having such a small particle size typically requires relatively more surfactant than an analogous emulsion in order to produce the necessary reduction in interfacial tension and increase in interfacial area.
Microemulsions and emulsions (or macroemulsions) differ in several ways. Typically, microemulsions form spontaneously under appropriate conditions and are thermodynamically stable. Furthermore, any phase separation resulting from freezing or a change in storage temperature is reversible, in that the microemulsion will reform when the original storage conditions are restored with perhaps some minimal agitation. Conversely, emulsions typically require additional energy to form, and will increase in particle size with time until the two immiscible components separate to their more thermodynamically preferred state.
Microemulsions may be oil- or water-continuous or may be bicontinuous. An oil-continuous system consists of water dispersed in oil, while a water-continuous system consists of oil dispersed in water. A bicontinuous system has no single dispersed phase and is typically considered to be a series of intertwining networks of the two phases.
Microemulsions generally maintain the viscosity of their continuous phase. In certain regions of the phase diagram, more complex micellar and other structures occur that lead to gels or highly viscous systems. Such systems can be similar to the bicontinuous structure mentioned above, or liquid crystalline gels, bilayered systems, rodlike or wormlike micelles, and others. In certain regions of phase space microemulsions can exist in equilibrium with other phases such as liquid crystalline phases.
Typically, systems that contain a fluorinated oil require relatively large amounts of a fluorinated surfactant in order to produce a microemulsion because such fluorinated surfactants are not very efficient at emulsification. This generally leads to a smaller one-phase region in the phase diagram wherein the microemulsion will form.